1. Field of the Invention
The present invention relates to a viscosity index improver, an engine lubricant composition, and a concentrate. More precisely, it relates to a viscosity index improver producing reduced amounts of coked products, an engine lubricant composition comprising said viscosity index improver and base oil, and a concentrate comprising an increased amount of said viscosity index improver and base oil.
2. Description of the Prior Art
With the recent tendency toward the global environmental protection, there is being an increased demand for much more improvements in fuel-related performance characteristics of cars (fuel consumption in cars). As one means of improving such fuel-related performance characteristics of cars, so-called multi-grade oil comprising engine oil and a viscosity index improver may be used. At present, about 30% of engine oil to be used in diesel engine cars in Japan will be such multi-grade oil. Multi-grade oil of the type generally contains a viscosity index improver comprising an ethylene-propylene copolymer (hereinafter referred to as "OCP viscosity index improver").
Where further improvements are required in fuel-related performance characteristics of cars in future, engine oil to be used will be required to have a reduced viscosity. In order to satisfy this requirement, engine oil must have a high viscosity index while having a lowered high-temperature high-shear viscosity (that is, a lowered TBS viscosity).
However, engine oil containing such an OCP viscosity index improver added thereto is defective in that its viscosity index is low while its high-temperature high-shear viscosity is high, thereby resulting in that the engine oil containing it is problematic in that its viscosity could not be lowered any more and that fuel-related performance characteristics of cars that use it could not be improved any more.
In order to solve these problems, the addition of a viscosity index improver containing an alkyl methacrylate polymer (hereinafter referred to as "PMA viscosity index improver") to engine oil may be taken into consideration. Since engine oil containing such a PMA viscosity index improver may have a higher viscosity index and a lower TBS viscosity than that containing an OCP viscosity index improver, it is expected that the former may have a lowered viscosity and that fuel-related performance characteristics of cars that use the former could be much more improved.
However, engine oil containing a PMA viscosity index improver is problematic in its practical use in that it gives large amounts of coked products.